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Overall tree and leaf unit transpiration variations in the Acacia tortilis under deficient rainfall conditions Volume 14, issue 4, OCTOBRE-NOVEMBRE-DÉCEMBRE 2003

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Laboratoire d‘écologie végétale, IRD, BP 1386, Dakar Sénégal <diagnealdakar.ird.sn>

In the Ferlo area, in Northern Senegal, seasonal transpiration variations at both tree and leaf levels are characterized and compared under favourable and deficient rainfall conditions (268 mm\year and 146 mm\year, respectively). Meanwhile, several relations are established using a few factors from the soil‐plant‐atmosphere system (soil water content in the upper layers, leaf phenology, and evaporative demand). Annually, the amount of water transpired by leaf unit area is 295 L.m‐ 2 in favourable periods and 300 L.m‐ 2 in deficient periods. Leaf transpiration is not affected by unfavourable rainfall conditions. The influence of deficient vapour pressure on the seasonal variation of single leaf transpiration is discussed. At tree level, the intensity of the transpiratory flow (delta) as estimated by the difference between the reference and minimal leaf water potentials shows a 30% decrease while the duration of the full foliation phase is shortened during the deficient year. The single leaf transpiration which is quantitatively the same under favourable and deficient conditions implies that the reduction of the whole tree transpiration is due to a decrease in the whole tree foliage area. Owing to its ability to adapt to inter‐annual rainfall fluctuations, this species should be recommended in the afforestation programs developed in the Sahel.